We have been to this party for the last 5 years. He will not be out of place but take his own feelings into consideration. My oldest was 10 the first time we went. First year no one wore costumes because we didn't know what to expect. Second year, when he was 11, he wore one only because little brother (5) wore one. He took it off mid way through the party. Third year, age 12, NO WAY.....not even with little brother wearing one. Halloween t-shirt was good enough for him. AND you can still trick or treat with or without the costume.
It really depends on your child. Some will love to continue to wear costumes and some will not. I would just make sure that it is easy to remove if he doesn't want to wear it anymore. My tween was all about the rides. If that's the same for you.....make sure that if he wears a costume it is also easy to get on and off the rides with. Even our youngest would shed his costume about midway through because he was either too hot, too uncomfortable or it was too difficult on the rides.